This invention relates to electric arc welding and more particularly to narrow groove welding.
In narrow groove welding, side walls of the welded joint, usually extremely thick plates, act as massive chills frequently making it difficult for the weld bead to fuse to the sides, causing defects related to a lack of fusion. Normally prescribed remedies such as increased heat input from a single welding torch or an array of welding torches have limited effectiveness.
In a hot wire tungsten inert gas welding process, side wall wetting can be improved if the arc established between a non-consumable tungsten electrode and the workpiece is oscillated to direct more of the arc energy to the sides of the groove. Several approaches to arc oscillation have been suggested or tried.
Magnetic pole pieces that can be programmed to produce a magnetic field of varying intensity and direction to cause oscillation of the welding arc have been used for some time. However, the use of this approach in narrow groove welding presents a space problem. The necessity of the pole piece being near the arc causes crowding within the joint. Further problems associated with heating of the pole pieces, disturbing influences of the magnetic forces created during welding of materials with high magnetic permeability, blocking of visual access to the weld pool, and difficulties in automatic monitoring have limited the desirability of this approach.
Magnetic oscillation of an arc can be caused through the use of twin electrodes which carry out of phase alternating currents. The practicality of this approach is limited by the fact that the frequency of oscillation will be relatively high, usually 60 Hz, and constant, and alternating current tends to increase the rate of deterioration of the tungsten electrode.
Conduction of the welding current away from the weld zone can be alternated between two or more grounds thereby causing the arc to oscillate under the changing influence of the magnetic field formed around the current path. This approach requires that the relative positions of the grounds and the arc remain constant, leading to the necessity of having the two grounds slide along the weld. Although such a scheme may work for weldments of simple geometry, it is mechanically too cumbersome for most welding jobs. Once again, crowding of the weld area may be a problem.
Some welding equipment utilizes an oscillating electrode assembly. Both lateral oscillation and oscillation about a pivot point have been used. An example of the pivot point oscillation technique is disclosed in U.S. Pat. No. 3,396,263, issued Aug. 6, 1968 to T. M. Evan et al.
Lateral oscillation and pivoting around a point produce the following arc sweeps respectively: EQU S.sub.L =W-2X-D (1) ##EQU1## where (as shown in FIGS. 1A and 1B) S is the sweep distance for the hottest point under the arc, W is the distance between the plates being welded, D is the torch assembly diameter, X is the minimum distance between the torch assembly and the joint walls, h is the height of the tapered section of the electrode, l.sub.o is the arc length, and b is the pivot point distance from the weld root.
Since the torch assembly usually serves to hold the electrode in place, effectively chill it and supply shielding gas around it, the diameter D can be substantial. Although X can be kept to a small value, it necessarily has a finite value to protect the torch assembly from damage. Equations (1) and (2) inherently imply that W-2X must be much larger than D for effective oscillation. No oscillation is possible if W-2X=D. Since W must be kept to a minimum, existing mechanical oscillation techniques are inadequate for some applications.
A number of welding processes and machines have been developed for oscillating an electrode in a consumable electrode welding system. Examples of these electrode oscillating techniques can be found in U.S. Pat. No. 4,074,105, issued Feb. 14, 1978 to Minehisa et al.; U.S. Pat. No. 3,576,966, issued May 4, 1971 to Sullivan; U.S. Pat. No. 2,163,657, issued June 27, 1939 to Beckman; U.S. Pat. No. 3,035,156, issued May 15, 1962 to Staley; and U.S. Pat. No. 3,609,292, issued Sept. 28, 1971 to Arnoldy. However, these patents do not address a non-consumable electrode welding system.
The methods and apparatus of the present invention represent an improvement in prior art arc welding processes using non-consumable electrodes, such as a hot wire tungsten inert gas process, where space considerations limit electrode assembly movement. Arc sweep distance is increased by angular oscillation around the axis of a non-consumable electrode having a tip which is radially displaced from the axis of the electrode. Rotation of this electrode will produce an arc sweep equal to twice the radial displacement of the tip.
The arc welding method of this invention is particularly useful for narrow groove welding and includes the creation of an arc between a metal workpiece and a non-consumable electrode, such as tungsten, having a radially offset tip and the angular oscillation of this electrode. In addition, lateral or pivotal oscillation or both can be added to the angular oscillation to achieve a greater sweep distance. This lateral or pivotal oscillation adds movement which is perpendicular to the welding line.
Non-consumable electrodes in accordance with this invention are generally straight with a shaped tip at one end which is radially displaced from the central axis of the electrodes. The opposite ends of these electrodes are provided with means, such as a threaded stud, for attachment to another electrode made of a highly conductive material such as copper. This highly conductive electrode can be of a smaller diameter due to its low resistivity, thereby allowing a smaller torch diameter.